Surging is an unstable operating condition that occurs in compressors, including centrifugal compressors used in refrigeration systems. Such a condition can be caused by an increase or decrease in compressor discharge pressure or by a reduction in the flow of gas to the compressor. These events can be triggered by poor maintenance of the refrigeration system, failure of a system component, or human error. Excessive surging, either in number of occurrences or in magnitude, may result in damage or complete failure of the compressor. Surging also results in inefficiencies in operation of a refrigeration system that result in excessive power consumption.
Extreme surging may be detectable by inspection of an operating compressor, by those knowledgeable in the art, but a compressor can operate in a surge condition with little vibration experienced. Different methods of detecting surge conditions in centrifugal compressors are known in the art. One method of detecting surge in a compressor is to monitor vibration of the compressor by mounting a vibration detector on or near the compressor to sense vibration caused by the compressor in a surged condition. Shortcomings of this method include the need for an extremely sensitive vibration sensor and false surge indications during start-up of the compressor.
Another method of detecting surge is by monitoring flow and pressure differences in the vicinity of the compressor as disclosed in U.S. Pat. No. 3,555,844, which is incorporated herein by reference. An alternative means of detecting surge is disclosed in U.S. Pat. No. 2,696,345, which is incorporated herein by reference and teaches monitoring temperature upstream of the impeller to detect an increase in temperature that precedes major surging. That same patent discloses a method of detecting surge by monitoring temperature on the discharge side of an axial flow compressor. However, as noted in U.S. Pat. No. 4,363,596, monitoring temperature in the discharge is not effective in a refrigerant compressor because the discharge temperature of such a compressor will actually go down when the compressor is in surge, since the flow to the discharge is basically stopped.
U.S. Pat. No. 4,363,596 teaches a method of detecting surge by measuring a temperature rise beyond a predetermined value in a space in the impeller chamber of the compressor, exterior of the flow path of gas through the impeller. The specification states that the temperature rise, above the normal operating temperature, occurring when the compressor is surging is caused by the increased heat produced by reduced compressor efficiency and the inability of the reduced gas flow to remove the heat. The disadvantage of this approach is that it measures the temperature rise in one location inside the impeller chamber and does not take into account that the temperature at the location may change due to a change in the operation condition of the compressor even when there is no surge. For example, a start-up condition is likely to give a false surge reading.
In the system disclosed in U.S. Pat. No. 4,151,725, a control system effectively maximizes efficiency without encountering surge problems by monitoring the temperature of the refrigerant in the condenser discharge line, the temperature of the saturated refrigerant leaving the evaporator, the temperature of the chilled water discharged from the evaporator of the chiller, and the inlet guide vane position. Based on the foregoing four parameters and a set point temperature input, the control system described in U.S. Pat. No. 4,151,725 effectively regulates the refrigeration system by regulating the speed of the compressor and adjusting vane position. A person skilled in the art will recognize that the temperatures being measured are unlikely to be influenced by incipient surge.
U.S. Pat. No. 5,746,062 discloses the method of detecting surges in a centrifugal compressor via sensing suction and discharge pressures of the compressor. The same patent also discloses surge detection through monitoring of the current applied to the variable speed motor drive that drives the compressor. It will be readily apparent to one skilled in the art that a sudden change in the load on the system, not necessarily related to surge, could also influence the current applied to the motor thus increasing the likelihood of a false positive detection of surge. This patent also teaches utilizing both pressure sensing and current sensing techniques to detect a surge. U.S. Pat. No. 5,746,062 is incorporated herein by reference.
The existing methods for detecting surges in centrifugal compressors integral to refrigeration systems are concentrated on monitoring conditions in the proximity of the compressor. One of the disadvantages of such systems is that they can generate a high number of false positive readings on account of their being influenced by localized, transient effects that generally may not be indicative of surge.